Telephone system



Get. 30, 1923. 1,472,485

- J. B. MORRILL TELEPHONE SYSTEM Filed Dec. 11, 1920 2 Sheets-Sheet 1 ct. 3f) 1-923.

-M/LES'OF 57'ANOAHD CABLE L055 MILES OFSTANDAHD CABLE GAIN 1,472,485 J. B. MORRILL TELEPHONE SYSTEM Filed Dec. 11, 1920 2 Sheets-Shae 2 2 4 '6 a 10 I2 1416 m 20 221425 NO. or s ur/01v mom DISPATCHER.

Mme/77hr. Jam m BMW/77A its:

JOSEPH B. MORRILL, OE ST. LOUIS, MISSOURI, ASSIGHOR T WESTERN ELECTRIC COM- PANY, INCOREORATED, OF N YGBK Y., A CORPORATION OF. YORK.

svs.

Application filed Becember 11, 19 30. erial lto. $9,893.

To all whomz't may concern:

Be it known that I, Josnrrr B. 1.1: w I a citizen of the United States, residing at St. Louis, in the county of St. Louis,'State of Missouri, have inventedcertain new and lo terns having a plurality'of substation circuits simultaneously bridged across the line, such for example, as are used in train dispatching. it relates more particularly to a system of this character employing loaded lin v It is well known that when loadin coils are introduced into a telephoneline or increasing its transmission efliciency, the elec trical length of the line is very much increased. t has'beenfound that in aline having high impedance substations bridged across it and loading coils inserted, the characteristic impedance of the line is increased and standing waves are set up in the line which have a harmful efiefi on both the volume and quality of the transmission, differing in kind and "amount at the difierent way stations.

' It is an object of the invention to eliminote the standing waves above. referred to. To accomplish this, the line is terminated in an impedance equal to the characteristic impedance of the line. -When so terminated, the line behaves substantially as a line ct infinite length, and hence there are no stand= ing waves in it.

In a system used for train dispatch the terminal way station, i. e., the one farthest from the dispatchers substation, can be given an impedance equal to the impedance of the line, but preferably thisi lastway station is shunted by an impedance equal to the impedance of the line.

The invention further resides in an improved form of substation circuit for use in connection with loaded cables.

The invention further resides in the coin bination with a loaded line of a plurality of substations bridged thereacross at frequent intervals.

The invention will be more fully under stood from the following detailed description and claims taken in connection with the accompanying drawing, in whichFig. 1 rep resent's'diagrammatically a system employing the invention; Fig. 2 represents the preferred method of terminating the line with the desired impedance; and Figs. 3 and 4 p are diagrams representing transmission conditions when the line is not properly terminated and when properly terminated, respectively. I Referring to the drawings. by reference characters, a series of waystation circuits 8,, S S,-S,, and S are connected across the line at frequent intervals. The dispatchers substation circuit DS, shown at the left, is substantially identical with the circuit disclosed in the application of l D. Edwards, Serial No. 315,753 filed August 6, 1919, and need not be described in detail The line, which may he a cable line, is provided at regular intervals with loading coils 10. The way stations, with the exception of the last, one, 8,, are identical, and a description of the substation S will sufiice' as a description of the others. A transformer 15 comprising line windings 16 and l? and local windings 18 and 19, and havmg a, core coon to said windings not shown, is employed to connect the receiver 20 and transmitter 21 to the line. A switch 25 is provided, which in the position shown open-circnitsthe transmitter, connects. the receiver in. series with the windings 18 and 19 and a condenser 28, and connects the windings 16 and 17 in series with a condenser. 30. en the switch is thrownto its opposite position, the transmitter with hattery 31 and the receiver are connected in parallel across winding 18, windings 17 and 19 are cut out of the circuit, and winding 16 is connected across the line through. a condenser 32.

The substation circuit is similar in general to that disclosed in the application of G. D. Edwards, Serial No. 289,667, ed April 12, 1919, but is an improvement thereon in that the condenser 28 1s connected in series with the receiver when the oircuit isin normal receiving condition, and

separate condensers 30 and 32 are connect- .ed in series withthe line winding in the receiving and talking conditions respeetoe isfactory results with medium loaded cables having up to thirty substations and ranging from No. 19 gauge wire for lines up to 40 miles long 'to No. gauge wire for lines up to 200 miles long. Medium loaded lines employ inductances of the order of .175 henries at a spacing of 1.66

miles. A substation best adapted for lines of No. 13 to No. 16 gauge wire, but working efliciently in the range above noted, employs a transmitter having an impedance of from 15 to 20 ohms, a receiver having an impedance of about 250 ohms at 800 cycles, a transformer having a core of laminated silicon steel, and windings 16, 17, 18 and 19 having respectively .2900 turns of No. 32 gauge wire, 7 600 turns of No. 36 gauge wire, 270 turns of No. 23 gauge wire, and 500 turns of No. 26 gauge wire, and direct current resistances respectively of 120, 1175, 1, and 5 ohms, and condensers 28, 30 and '32 having the values respectively of .67, .009 and .25 microfarads.

These values are given by way of example only, it being understood that the desired substation impedance can be obtained in I a variety of ways. Best results are obtained when the substation in its receiving condition has an impedance of approximately 35,000 ohms with an impedance angle of 20 at 800 cycles and an impedance of approximately 20,000 ohms with an impedance angle of +45 at- 1500 cycles. This corresponds to the impedance of the substation given as an example above.

If the line were terminated in a fina substation S having an impedance identical with the other substations, standing waves would be set up in the line as indicated in Fig. 3. The curves in this diagram were obtained by computations for each of the 25 equally spaced substations in a medium loaded line of No. 13 gauge wire 100 miles long. It will be noted from Fig. 3 that at the 24th substation, there is a loss of 26 miles of standard cable at 800 cycles, while at 1500 cycles the loss is only about 9 miles. This,

across the line terminals. hae been confirmed experimentally.

if not corrected, would, of course,- cause considerable distortion of the speech sig nals at this substation. The curves of Fig. 1 were obtained from the same line after an impedance equal to the characteristic impedance of the line had been connected Theseresults One way of providing the terminal impedance is shown in Fig. 1, in which the windings 16 and 17 have been modified so that the substation has the impedance to properly terminate the line. It has been found preferable,- however, to employ a separate impedance as shown at 4:0 in Fig. 2 in shunt to the final waystation, in which case the circuit of Substations S is identical with the preceding waystation circuits. Since, in this case, the impedance of substation S corresponds to that of substation 8,, as described above, this impedance is very high in comparison with that of the terminating impedance 10, which is a resistance of the order of 1200 ohms. Since each of the substations is shunted by the characteristic impedance of the line, which is equal to that of the element 40, the use of the latter in shunt to the final station S produces very little loss in 55i In fact, by reference to Figs. 3 and 4., it will be seen that there is not only amarked improvement in quality, but also on the average of the twenty-five stations, there is a slight improvement in the quantity of transmission by the use of a proper termination.

Electrical Engineering Problems, page r-l-jlw i/ H in which Z is the characteristic impedance, and 7, Z, 9 and a are respectively the reslstance, inductance, conductive leakance,

and capacity per unit length of line and m is the frequency multiplied by 2i:. For most loaded cables the impedance angle is sufficiently smallthat resistance 40 may be are-resistance. Since, in loaded cables, the istributed conductive leakage is negligible,

the conductance of the substations, if nearly equally spaced, may be regarded as uniformly distributed and this is also true of the inductance of the loading coils.

What isclaimed is .1. The combination with a line having series impedances and substations connected in shunt at frequent intervals, the in-.

dividual impedances of such substations being high relative to that of said line, of

a terminal device having an impedance pqual to the characteristic impedance of the 2. The combination with a line having a plurality of substations in bridge thereof, the electrical length of said line being such that standing waves tend to interfere with transmission, of means to prevent the formation of standing waves.

3. The combinatibnwith a line having a plurality of'substations in bridge thereof,

the electrical length of said line being such that standing waves tend to interfere with transmission, of a terminating impedance the value of which is so related to the characteristic impedance of the line as to prevent the formation of standing waves. I

4. The combination with a line having a plurality of substations bridge thereof,

the electrical length of said line being such that standing waves tend to interfere with transmission, of a terminating impedance to prevent the formation of standing waves,

v said impedance being equal to the characteristic impedance of said line.

' 5. In combination, a telephone line and a plurality of substations in bridge thereof, one of said substations comprising a receiver, a transformer adapted to connect said receiver across a line, and a condensive reactance associated with- "said transformer, said elements being so designed as to give said substation an impedance of about 35,- 000 ohms with an impedance angle of about 20 at a frequency of 800 cycles.

6. In combination, a telephone line and a plurality of substations in bridge thereof, one of said substations comprising a receiver, a transformer adapted to connect said receiver across a line, and a condensive reactanoe associated with said transformer,

said elements being so designed as to give said substation an impedance of about 35,- 000 ohms with an impedance angle of about 20 at a frequency of 800 cycles and an impedance of about 20,000 ohms with an impedance angle of +45 at a frequency of 1500 cycles.

7 A telephone substation comprising a transformer having a plurality of line windings, a plurality of condensers differing in capacity, and switching means for connecting one ofv said windings in series with one of said condensers across a line and for connecting a plurality of said windings in series with another of said condensers across the line. i

8. The combination with a telephone line and a substation bridged across said line intermediate the ends thereof, the electrical length of said line and the position of said substation being such that standing waves tend to distort complex waves at said substation, of a terminating impedance to prevent the formation of standing waves, said impedance being equal to the characteristic impedance of said line.

In witness whereof, I hereunto subscribe my name this 29th day of November A. 1).,

JOSEPH B. Momma. 

